1. Field of the Invention
The present invention relates to a fixing apparatus used in an image forming apparatus such as a copy machine, a printer or a facsimile machine and, more particularly, to a belt-type fixing apparatus in which a heat generated by a heat roller is transferred to a nip portion between a fixing roller and a pressing roller via a fixing belt.
2. Description of the Related Art
In an image forming apparatus such as a copy machine, a printer or a facsimile machine,:a toner image is formed on a image carrier such as a photosensitive drum or an intermediate transfer belt. The toner image formed on the image carrier is transferred to a print paper or an OHP sheet by a transfer means, and the transferred toner image is fixed by a fixing apparatus. Conventionally, a heat roller fixing apparatus is used for such a fixing apparatus. In the heat roller fixing apparatus, a pressing roller is pressed against a fixing roller which is heated by a heater incorporated in the fixing roller. The pressing roller and the fixing roller are rotated in reverse directions by a drive mechanism. A print paper is conveyed through a nip portion formed between the pressing roller and the fixing roller so that a toner image on the print paper is fixed by a heat provided from the fixing roller and a pressing force applied by the pressing roller.
Japanese Laid-Open Patent Application No. 4-322282 discloses a technique for preventing generation of wrinkles in a print paper when the print paper is subjected to a fixing process in an image forming apparatus. This technique is used in a heat roller fixing apparatus comprising a pair of a fixing roller and a pressing roller. In this technique, generation of wrinkles is prevented by reducing a hardness of a conductive foam layer of the pressing roller.
In the above-mentioned heat roller fixing apparatus, there is a limit to increasing a width of the nip portion between the fixing roller and the pressing roller. In order to fix a toner image on the print paper at a low temperature, a duration (may be referred to as a nip period) of the print paper being positioned in the nip portion should be increased. That is, in order to achieve a complete fixation at a low temperature, a rotational speed of the pressing roller and the fixing roller must be reduced since the width of the nip portion cannot be increased. Thus, it is difficult to achieve a high-speed fixation.
An outer surface of the fixing roller is maintained at a high temperature due to heat provided from a heater incorporated in the fixing roller. Accordingly, a rate of increase in a temperature of the toner is high due to the increase in a nip period during which the fixing belt is located in the nip portion. This results in a decrease in a hot offset temperature which is an upper limit of a fixing temperature of the toner. The hot offset temperature is a temperature at which the hot offset phenomenon occurs. Accordingly, there is a problem in that an allowable range of a temperature for fixing the toner with a good exfoliation is decreased. The allowable range is referred to as an exfoliative fixation temperature range.
Accordingly, in order to eliminate the above-mentioned problem, Japanese Laid-Open Patent Application No.6-318001 discloses a belt-type fixing apparatus for an electrophotographic apparatus using a fixing belt. The belt-type fixing apparatus disclosed in this patent document comprises a fixing roller and a heat roller and an endless fixing belt drivingly engaged with the fixing roller and the heat roller. The fixing roller is provided with an exfoliative layer. A pressing roller is pressed against the fixing roller via the fixing belt interposed therebetween so that a nip portion is formed between the fixing belt and the pressing roller. A heating passage which extends substantially along a straight line is formed between the fixing belt and a recording medium supporting member which is provided under the fixing belt between the heat roller and the nip portion. A specific heat of the fixing belt is set to 0.002 to 0.025 cal/xc2x0 C./cm2.
In the above-mentioned belt-type fixing apparatus, a fixing temperature at the nip portion can be reduced due to a pre-heating effect provided to a toner image by the heating passage. Additionally, a temperature of the toner image is not excessively raised due to a self-cooling effect of the fixing belt having a small heat capacity. Thus, an exfoliation of the toner image from the fixing belt is improved, which results in. a complete fixation of the toner image without an offset even when no oil or a very small amount of oil is applied to the fixing belt.
A description will now be given, with reference to FIGS. 1 and 2, of advantages of the belt-type fixing apparatus over the heat roller fixing apparatus. FIG. 1 is an illustration showing a temperature distribution of each of a fixing roller, a toner image and a print paper. In a heat roller fixing apparatus, a fixing roller 21 comprises a metal core 21a and a silicon rubber layer 21b provided on the metal core 21a. On the assumption that a constant amount of heat is provided from a heater inside the metal core 21a in the fixing roller 21, the fixing roller is initially maintained at a constant temperature T0. A print paper 22 and a toner layer 23 forming the toner image on the print paper 22 are also maintained at a temperature Ta which is normally a room temperature (about 25xc2x0 C.).
Immediately after the print paper 22 and the toner layer 23 enter the nip portion formed between the fixing roller 21 and the pressing roller 24, a temperature change from the fixing roller 21 to the print paper 22 occurs as indicated by a temperature distribution curve t1. As time passes, the distribution of temperature is changed as indicated by curves t2 and t3. At this time, an interface between the silicon rubber layer 21b of the fixing roller 21 and the toner layer 23 of the print paper 22 maintains a constant boundary temperature T1 (corresponding to a temperature of an upper surface of the toner layer 23).
On the other hand, as time passes, a heat is transferred from the fixing roller 21 to the toner layer 23. Thus, a boundary temperature Tf (corresponding to a temperature of a lower surface of the toner layer 23) between the toner layer 23 and the print paper 22 is increased. If the nip period is sufficiently long, the temperature distribution becomes a curve t4, and the boundary temperature T1 is increased and the boundary temperature Tf is also increased.
In the belt-type fixing apparatus, if a heat source is not provided within the nip portion, a temperature of the surface of the fixing belt is decreased as time passes since a heat is removed by the print paper through the surface of the fixing belt. This is referred to as a self-cooling effect of the fixing belt. Accordingly, the boundary temperature Tf is increased as time passes. However, as long as the boundary temperature T1 is concerned, the boundary temperature T1 does not sharply increase as is in the conventional heat roller fixing apparatus.
A hot offset phenomenon occurs at the interface between the fixing roller 21 and the toner layer 23 when an interface adhesive force of the toner layer 23 exceeds a cohesive force related to a viscoelasticity when the toner is melted. That is, the hot offset phenomenon is influenced by the level of the boundary temperature T1 which is a temperature of the interface between the silicon rubber layer 21b of the fixing roller 21 and the toner layer 23. Additionally, a fixation can be achieved when the interface adhesive force exceeds the cohesive force of the toner which is related to the viscoelasticity of the toner when the toner is melted. That is, the fixation is influenced by the level of the boundary temperature Tf which is a temperature of the interface between the toner layer 23 and the print paper 22.
FIG. 2 is a graph showing fixation temperature characteristics of the heat roller fixing apparatus and the belt-type fixing apparatus. In FIG. 2, the horizontal axis represents a nip period which is a period during which the fixing belt is located in the nip portion, and the vertical axis represents a fixing temperature. A surface temperature of the fixing roller of the heat roller fixing apparatus is indicated by a fine solid line, and a surface temperature of the fixing roller of the belt-type fixing apparatus is indicated by a bold solid line. Additionally, a temperature of the toner of the heat roller fixing apparatus is indicated by a fine dashed line, and a temperature of the toner of the belt-type fixing apparatus is indicated by a bold dashed line.
As interpreted from FIG. 2, in a range (fixable range) of the temperature in which the. toner is fixable, the temperature in the heat roller fixing apparatus rapidly increases as the nip period is increased, whereas an increase in the boundary temperature T1 can be prevented in the belt-type fixing apparatus due to the self-cooling effect of the fixing belt which results in a gentle increase of the temperature of the toner. Accordingly, when the nip time can be increased, the belt-type fixing apparatus has a greater allowance than the heat roller fixing apparatus with respect to the offset of the toner.
In the above-mentioned conventional belt-type fixing apparatus, since the pre-heating process of the toner is performed by heating an atmosphere of the toner, there is a problem that a sufficient pre-heating cannot be achieved when the pre-heating process must be performed at a high speed. Additionally, since the fixing roller is made of a solid rubber, a stress due to a thickness of the print paper cannot be absorbed which results in an uneven distribution of a pressure in which the pressure at an entrance of the nip portion is low and a pressure in the middle of the nip portion is high. Accordingly, there is a problem in that wrinkles are generated in the print paper because the print paper, which has been heated in the pre-heating process resulting in a generation of a partial unflatness due to evaporation of water contained in the print paper, is pressed by a large surface pressure in the nip portion having an uneven pressure distribution.
Additionally, in the above-mentioned belt-type fixing apparatus, there is a problem in that a recovery of a temperature of the surface of the fixing belt to a predetermined fixing temperature takes a long time since a heat roller is the only means for heating the fixing belt and the temperature of the surface of the fixing belt is considerably decreased when the fixing belt passes through the nip portion.
It is a general object of the present invention to provide an improved and useful fixing apparatus in which the above-mentioned problems are eliminated.
A more specific object of the present invention is to provide a fixing apparatus in which a generation of wrinkles in a print paper can be prevented when the print paper is passed through a nip portion between a fixing belt and a pressing roller.
Another object of the present invention is to provide a fixing apparatus in which a print paper is positively separated from the fixing belt after the print paper and the fixing belt exit the nip portion.
A further object of the present invention is to provide a fixing apparatus in which a temperature of the fixing belt can be rapidly recovered after the fixing belt passes through the nip portion and the temperature of the fixing belt is decreased.
In order to achieve the above-mentioned objects, there is provided according to one aspect of the present invention a fixing apparatus for fixing a toner provided on a recording sheet, the fixing apparatus comprising:
a fixing roller;
a heat roller generating heat;
an endless fixing belt drivingly engaged with the fixing roller and the heat roller; and
a pressing roller pressed against the fixing roller via the fixing belt so that a fixing process area is formed between the fixing roller and the pressing roller, the recording sheet being passed through the fixing process area so that the toner provided on the recording sheet receives a heat from the fixing belt,
wherein the fixing roller includes a foamed material layer elastically deformable by the pressing roller.
According to the above-mentioned invention, the fixing roller has a foamed material layer which elastically deforms when the pressing roller is pressed against the fixing roller. The foamed material has a relatively low hardness as compared to a solid rubber material. Thus, a surface pressure in the fixing process area (nip portion) between the fixing roller and the pressing roller is substantially uniform and low over an entire length of the fixing process area. Thus, a generation of wrinkles in the recording sheet can be prevented when the recording sheet entering the fixing process area has a waving part. Additionally, the foamed material layer of the fixing roller serves as a heat insulating material to prevent a release of heat from the fixing process area. Thus, a sufficient amount of heat can be provided to the toner on the recording sheet while the recording sheet passes through the fixing process area.
In one embodiment of the fixing apparatus according to the present invention, the foamed material layer of the fixing roller may have a hardness ranging from 10xc2x0 to 50xc2x0 measured by Asker C. Additionally, the foamed material layer may be made of a foamed silicon rubber. The pressing roller may also include a foamed material layer which is preferably made of a foamed silicon rubber.
Additionally, a hardness of the foamed material layer of the fixing roller may be lower than a hardness of the pressing roller. When the hardness of the fixing roller is lower than the hardness of the pressing roller, the fixing roller is elastically deformed by the pressing roller so that the deformed portion of the fixing roller in the fixing process area substantially follows a shape of the pressing roller. Thus, the recording sheet exiting from the fixing process area is moved in a direction in which the recording sheet is positively separated from the fixing belt which follows an outer surface of the fixing roller. Thus, an undesired adherence of the recording sheet to the fixing belt, which results in an occurrence of an excessive melt and a hot offset of the toner, can be prevented.
In one embodiment of the fixing apparatus according to the present invention, the pressing roller may pressingly contact the fixing belt before the fixing process area is formed so that a pre-heating area is formed as a contact area of the pressing roller and the fixing belt.
Additionally, there is provided according to another aspect of the present invention a fixing apparatus for fixing a toner provided on a recording sheet, the fixing apparatus comprising:
a fixing roller;
a heat roller generating heat;
an endless fixing belt drivingly engaged with the fixing roller and the heat roller;
a pressing roller pressed against the fixing roller via the fixing belt so that a fixing process area is formed between the fixing roller and the pressing roller, the recording sheet being passed through the fixing process area so that the toner provided on the recording sheet receives a heat from the fixing belt; and
a heat conductive member having a first end surface and a second end surface remote from the first end surface, the first end surface contacting the heat roller, the second end surface contacting the fixing belt.
According to the above-mentioned invention, heat generated by the heat roller can be transmitted to a position remote from the heat roller through the heat conductive member. Thus, if the second end surface of the heat conductive layer is. in contact with a portion of fixing belt having a decreased temperature, a recovery time of temperature of the fixing belt can be reduced.
In one embodiment of the present invention, the heat conductive member may be positioned inside of a loop formed by the fixing belt, and the second end surface of the heat conductive member contacts an inner surface of the fixing belt.
Additionally, the heat conductive member is made of an elastically deformable material so that the second end surface of the heat conductive member always contacts the fixing belt even when a slack is generated in the fixing belt.
In one embodiment of the present invention, a width of a contact area of the heat conductive member to the fixing belt is variable in response to a width of the recording sheet, the width being measured in a direction perpendicular to a moving direction of the fixing belt. Accordingly, a non-contact portion of the fixing belt which does not contact the recording sheet can be prevented from being heated by the heat conductive member. Thus, an excessive heating of the non-contact portion can be prevented.
Additionally, the fixing apparatus according to the present invention may further comprise a heat insulating material covering surfaces of the heat conductive material other than the first end surface and the second end surface.
In this invention, the heat insulating material prevents a release of heat from the heat conductive member. Thus, the heat is efficiently transmitted from the heat roller to the fixing belt. Additionally, since a heat is not released from the heat conductive member which may positioned adjacent to a portion of the fixing belt which has been heated at an appropriate temperature, an influence of a heat released from the heat conductive member can be eliminated.
Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings.